What does an extract statement do in the ABAP program?

Once you have declared the possible record types as field groups and defined their structure, you can fill the extract dataset using the following statements:
EXTRACT <fg>.

When the first EXTRACT statement occurs in a program, the system creates the extract dataset and adds the first extract record to it. In each subsequent EXTRACT statement, the new extract record is added to the dataset

EXTRACT HEADER.
When you extract the data, the record is filled with the current values of the corresponding fields.

As soon as the system has processed the first EXTRACT statement for a field group <fg>, the structure of the corresponding extract record in the extract dataset is fixed. You can no longer insert new fields into the field groups <fg> and HEADER. If you try to modify one of the field groups afterwards and use it in another EXTRACT statement, a runtime error occurs.

By processing EXTRACT statements several times using different field groups, you fill the extract dataset with records of different length and structure. Since you can modify field groups dynamically up to their first usage in an EXTRACT statement, extract datasets provide the advantage that you need not determine the structure at the beginning of the program.